Sure they can. They can 'dense up' into what are being called 'edge cities,' which still can have some of the bad implications of suburbs, such as economic and/or racial segregation. Even better, there is vacant or underutilized land in cities themselves, contrary to a remark I dimly remember in the thread. To some extent, this is already happening as the costs of suburbanization mount and begin to become noticeable to suburbanites, such as the cost of new infrastructure, time required for commuting, etc.
MBS